During locomotion in persons with post-stroke hemiparesis, muscle activity is initiated at inappropriate points in the cycle. As a consequence, movements are less-forceful and are slower, and movement function is impaired. The investigators propose that an interaction between two key underlying mechanisms, heightened motoneuron excitability and abnormal position-dependent modulation of motoneuron excitability, result in inappropriately-timed muscle activity. With their earlier work, the investigators have shown that paretic uniarticular knee muscles and biarticular hip and knee muscles are inappropriately activated at an earlier phase in the pedaling cycle. Since these muscles are lengthening at these points in the cycle and, since this effect is speed-dependent, they first propose that heightened motoneuron excitability results in muscle being activated when it is stretched at a specific threshold length and velocity. They will systematically vary the ranges of length and velocity of uniarticular knee muscles during cyclical leg motion to identify threshold muscle stretch parameters that trigger inappropriate initiation of uniarticular muscle activity. They will also systematically vary the ranges of length and velocity of uni- and biarticular muscles crossing the hip to identify muscle stretch parameters that, secondarily, contribute to inappropriate initiation of uniarticular knee muscle activity. They will use a computer model of the musculoskeletal system to calculate each muscle's length and velocity characteristics from kinematic patterns and develop a comprehensive statistical model of the relative contributions from multiple muscle stretch parameters. Also, normally during cycling, uniarticular knee extensors are activated during knee extension, regardless of hip position. However, preliminary work in post-stroke subjects has demonstrated abnormal activation that is dependent on hip position. They propose that the position of the hip can abnormally modulate motoneuron excitability and, hence, influence timing of muscle activity in uniarticular knee extensor muscles. They will systematically vary the relative position of the hip versus knee using a unique linkage attached to the feet. This experiment will result in kinematic patterns that generate more appropriate timings of uniarticlar knee extensors. The intent is that the experimental apparatus and principles developed within this study will form the basis of a new therapeutic modality that targets deficits in locomotor control, post-stroke, and with other neurologic conditions.